spawnInstance now sets DISABLE_RATE_LIMITS=1 by default so unrelated tests
don't exhaust the shared 127.0.0.1 per-IP bucket. bootHomePlusRemotes/
bootTwoInstances accept an optional { enableRateLimits } that omits the env
for tests that need real enforcement, exposed via the explicit
bootTwoInstancesWithRateLimits() helper for Test #15 (rate-limit assertion).